Field of the Invention
The present invention relates to a mode scrambler and, more specifically, to a mode scrambler capable of rapidly scrambling the mode distribution of light transmitted through an optical fiber to establish a stationary mode distribution.
In measuring the optical transmission loss characteristics and transmission band characteristics of an optical fiber in the field of optical fiber communication or optical fiber measurement, particularly, in the field of optical fiber measurement, it is essential to the standardization of measuring conditions that the incident light falling on the measuring instrument is in a stationary mode distribution. A stationary mode distribution is a state in which the mode distribution of light transmitted through an optical fiber is a certain outgoing distribution independent of the conditions of the incident light and axial position in the optical fiber.
A certain distance is necessary to establish a stationary mode distribution in a multimode optical fiber. A long distance as long as ten meters or longer is necessary to establish a stationary mode distribution of transmitted light particularly in a plastic optical fiber. A mode scrambler has been developed to establish the stationary mode distribution of incident light emitted from a light source readily and at a low transmission loss.
A mode scrambler employing a quartz optical fiber having a small numerical aperture of less than 0.4 and wound around a plurality of cylinders for mode conversion is reported in Tokuda et al., Denki Tsushin Gakkai Hikari.Denpa Bumon Zenkoku Taikai Koen Ronbun-shu S3-9, 1976.
J.P. Provisional Pub. (Kokai) No. 60-178409 discloses a mode scrambler having a construction similar to the foregoing mode scrambler of Tokuda et al., employing an optical fiber longitudinally twisted and bent by a plurality of pins.
A further mode scrambler employing a quartz optical fiber wound helically around a cylinder has been proposed.
The mode scrambling performance of a mode scrambler of the conventional construction is effective when the optical fiber is a thin optical fiber having a core diameter of several tens micrometers or less, such as a quartz optical fiber. However, a conventional mode scrambler is unable to function effectively when the mode scrambler employs a thick optical fiber of 100 .mu.m or above in core diameter and 0.4 or above in numerical aperture, such as a step index optical fiber, and the conventional mode scrambler capable of satisfactory performance is inevitably very large.
In a mode scrambler comprising a cylinder and an optical fiber having a large core diameter, particularly, a plastic optical fiber having a large core diameter, wound helically around the cylinder, the mode distribution of the outgoing light is dependent on the diameter of the cylinder and the conditions of the incident light, and only light of higher mode or lower mode is distributed. Thus, this mode scrambler is unable to function satisfactorily.